You can spend any amount of money with these flushing compounds. Some of them will eat up your rad, your heater core, and once in a while a head gasket. The best and cheapest thing to use is Sal Soda, made by Arm&Hammer. Its nothing more than washing soda, one would have found in the wringer machine days. Probally has the same stuff in it that Bill posted about. I used it yrs ago, just put a couple of cups in plain water and drive the truck aound a 1000 miles or so. It cleans up all the junk left in the block and rad, and dont harm anything. A lot of times overheating in these engines is caused by crud build up in the water jacket located along the length of the left side of the block. The water doesn't flow though here well, and build up keep the water from drawing the heat out of the metal along bottom of the cylinder walls. This passage ends at the drain cock at the back of the block. Removing this plug and getting in there with a long brush, similar to what is used to clean a commerical coffee maker will help knock the stuff loose. There is also a frost plug at the front end of the motor that can be pulled, and the same thing done there. Makes a world of differance if you have constant overheating trouble.
